This invention relates to an electrically heatable laminated glazing, especially an automobile glazing.
The simplest laminated glazing includes two glass panes bonded together with a thermoplastic film of a material such as polyvinyl butyral which has excellent bonding properties. To provide an electrically heatable laminated glazing U.S. Pat. Nos. 4,654,067, 4,668,270, and 4,744,844, describe such a laminated glazing which further includes a layered construction positioned between one of the glass panes and the thermoplastic film comprising an anti-reflective layer of zinc oxide and a conductive silver layer disposed on the zinc oxide layer. Optionally, an additional anti-reflective layer of zinc oxide is disposed on the silver layer, forming a zinc oxide-silver-zinc-oxide construction, which is again positioned between one of the glass panes and the thermoplastic film. This layered construction is applied to the glass with magnetic field-supported cathode sputtering or magnetic field-supported reactive cathode sputtering.
Also known to the prior art is the use of additional layers in the layered construction including a thin layer of metal or an under-oxidized (i.e., substoichiometric) metal oxide disposed between the silver layer and the anti-reflective covering layer, as disclosed in European Patent No. 0 104 870. The purpose of the thin metallic layer is to protect the silver layer from the influence of oxygen during the cathode sputtering of the anti-reflective covering layer.
European Patent No. 0 263 623 discloses a laminated glazing including a layer for improving the bond to the polyvinyl butyral film disposed on the anti-reflective covering layer of zinc oxide. In this known heatable laminated glazing, a layer of chromium oxide is provided to improve the bond, wherein the chromium oxide layer has a thickness of approximately 25 to 100 Angstroms, and consists of a mixture of metallic chromium and chromium oxides of varying degrees of oxidation (i.e., CrO, CrO.sub.2, CrO.sub.3, and Cr.sub.2 O.sub.3). This uppermost layer of chromium oxide is intended to solve the well-known problem of deterioration of the bond between the surface coating and the polyvinyl butyral film over the course of time and under the influence of UV radiation. In some instances, the deterioration can advance such that the required safety properties of the laminated glazing are jeopardized.